/*
 * Copyright (C) 2009 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Comparator;

import javax.annotation.Nullable;

import com.google.common.primitives.Booleans;
import com.google.common.primitives.Ints;
import com.google.common.primitives.Longs;

/**
 * A utility for performing a "lazy" chained comparison statement, which
 * performs comparisons only until it finds a nonzero result. For example:
 * 
 * <pre class="code">
 * {@code
 * 
 *   public int compareTo(Foo that) {
 *     return ComparisonChain.start()
 *         .compare(this.aString, that.aString)
 *         .compare(this.anInt, that.anInt)
 *         .compare(this.anEnum, that.anEnum, Ordering.nullsLast())
 *         .result();
 *   }}
 * </pre>
 * 
 * The value of this expression will have the same sign as the <i>first
 * nonzero</i> comparison result in the chain, or will be zero if every
 * comparison result was zero.
 * 
 * <p>
 * Once any comparison returns a nonzero value, remaining comparisons are
 * "short-circuited".
 * 
 * @author Mark Davis
 * @author Kevin Bourrillion
 * @since 2010.01.04 <b>tentative</b>
 */
@SuppressWarnings({ "rawtypes", "unchecked" })
public abstract class ComparisonChain {
    private ComparisonChain() {
    }

    /**
     * Begins a new chained comparison statement. See example in the class
     * documentation.
     */
    public static ComparisonChain start() {
        return ACTIVE;
    }

    private static final ComparisonChain ACTIVE = new ComparisonChain() {
        @Override
        public ComparisonChain compare(Comparable left, Comparable right) {
            return classify(left.compareTo(right));
        }

        @Override
        public <T> ComparisonChain compare(@Nullable T left, @Nullable T right,
                Comparator<T> comparator) {
            return classify(comparator.compare(left, right));
        }

        @Override
        public ComparisonChain compare(int left, int right) {
            return classify(Ints.compare(left, right));
        }

        @Override
        public ComparisonChain compare(long left, long right) {
            return classify(Longs.compare(left, right));
        }

        @Override
        public ComparisonChain compare(float left, float right) {
            return classify(Float.compare(left, right));
        }

        @Override
        public ComparisonChain compare(double left, double right) {
            return classify(Double.compare(left, right));
        }

        @Override
        public ComparisonChain compare(boolean left, boolean right) {
            return classify(Booleans.compare(left, right));
        }

        ComparisonChain classify(int result) {
            return (result < 0) ? LESS : (result > 0) ? GREATER : ACTIVE;
        }

        @Override
        public int result() {
            return 0;
        }
    };

    private static final ComparisonChain LESS = new InactiveComparisonChain(-1);

    private static final ComparisonChain GREATER = new InactiveComparisonChain(
            1);

    private static final class InactiveComparisonChain extends ComparisonChain {
        final int result;

        InactiveComparisonChain(int result) {
            this.result = result;
        }

        @Override
        public ComparisonChain compare(@Nullable Comparable left,
                @Nullable Comparable right) {
            return this;
        }

        @Override
        public <T> ComparisonChain compare(@Nullable T left, @Nullable T right,
                @Nullable Comparator<T> comparator) {
            return this;
        }

        @Override
        public ComparisonChain compare(int left, int right) {
            return this;
        }

        @Override
        public ComparisonChain compare(long left, long right) {
            return this;
        }

        @Override
        public ComparisonChain compare(float left, float right) {
            return this;
        }

        @Override
        public ComparisonChain compare(double left, double right) {
            return this;
        }

        @Override
        public ComparisonChain compare(boolean left, boolean right) {
            return this;
        }

        @Override
        public int result() {
            return result;
        }
    }

    /**
     * Compares two comparable objects as specified by
     * {@link Comparable#compareTo}, <i>if</i> the result of this comparison
     * chain has not already been determined.
     */
    public abstract ComparisonChain compare(Comparable<?> left,
            Comparable<?> right);

    /**
     * Compares two objects using a comparator, <i>if</i> the result of this
     * comparison chain has not already been determined.
     */
    public abstract <T> ComparisonChain compare(@Nullable T left,
            @Nullable T right, Comparator<T> comparator);

    /**
     * Compares two {@code int} values as specified by {@link Ints#compare},
     * <i>if</i> the result of this comparison chain has not already been
     * determined.
     */
    public abstract ComparisonChain compare(int left, int right);

    /**
     * Compares two {@code long} values as specified by {@link Longs#compare},
     * <i>if</i> the result of this comparison chain has not already been
     * determined.
     */
    public abstract ComparisonChain compare(long left, long right);

    /**
     * Compares two {@code float} values as specified by {@link Float#compare},
     * <i>if</i> the result of this comparison chain has not already been
     * determined.
     */
    public abstract ComparisonChain compare(float left, float right);

    /**
     * Compares two {@code double} values as specified by {@link Double#compare}
     * , <i>if</i> the result of this comparison chain has not already been
     * determined.
     */
    public abstract ComparisonChain compare(double left, double right);

    /**
     * Compares two {@code boolean} values as specified by
     * {@link Booleans#compare}, <i>if</i> the result of this comparison chain
     * has not already been determined.
     */
    public abstract ComparisonChain compare(boolean left, boolean right);

    /**
     * Ends this comparison chain and returns its result: a value having the
     * same sign as the first nonzero comparison result in the chain, or zero if
     * every result was zero.
     */
    public abstract int result();
}
